The present invention relates to a radiological imaging apparatus comprising semiconductor radiation detector modules.
Examples of medically applied radiation measurement systems include radiological imaging apparatuses such as a gamma camera, a single photon emission computed tomography (SPECT) apparatus, and a positron emission tomography (PET) apparatus.
In examinations using a radiological imaging apparatus, radiopharmaceuticals are administered to a subject that is a testing object; the radiopharmaceuticals contains a substance tending to deposit in a particular site (for example, cancerous cells). Then, radiation detectors detect radiations emanated from the subject's affected part as a result of the radiopharmaceuticals deposited in the site. The radiations emanated from the subject's affected part each have a fixed quantity of energy (for PET examinations, 511-KeV energy). On the basis of detection signals output by the radiation detectors having detected the radiations, the radiological imaging apparatus creates a tomographic image containing an image of the area in which the radiopharmaceuticals are deposited, that is, an image of the subject's cancerous part. On the basis of the created tomographic image, a medical doctor specifies a cancerous part.
A radiological imaging apparatus is known which uses semiconductor radiation detectors (referred to as semiconductor detectors below) as radiation detectors (see JP-A-2004-125524). The semiconductor detector is composed of any one of semiconductor materials such as CdTe (cadmium telluride), GaAs (gallium arsenide), and TlBr (thallium bromide). The semiconductor detector has electrodes installed on opposite surfaces of the semiconductor member. When a radiation is incident on the semiconductor member, the semiconductor detector generates electric charges corresponding to the energy of the radiation on the basis of photoelectricity. The electric charges generated by the semiconductor member are taken out of an electrode to which a charge collection bias voltage is applied. As a result, an electric signal is obtained. Thus, the semiconductor detector outputs the electric charges generated by the interaction between the radiation and the semiconductor material, as an electric signal. The semiconductor detector thus has an excellent energy resolution.
However, if the semiconductor detector is applied to the radiological imaging apparatus, measures must be taken for a phenomenon called polarization. The polarization appears significantly in semiconductor detectors using CdTe Schottky diodes. This phenomenon will be described. Continuous application of a bias (a reverse bias for normal diodes) voltage to the CdTe Schottky diode may degrade the energy resolution in several to several tens of minutes. Further elapse of time may degrade detection efficiency or photoelectric conversion efficiency. In this state, accurate measurements are impossible. The polarization results from the accumulation of space charges as shown in “Radiation Measurement Handbook, 3rd Edition written by KNOLL and translated by Itsuro KIMURA and Eiji SAKAI, THE NIKKAN KOGYO SHIMBUN LTD., p. 548”. Thus, when the bias voltage is zeroed simultaneously with the end of measurements, the space charges are recombined to avoid the polarization (see JP-A-2004-125524).
JP-A-2004-125524 describes a radiation detection system comprising a semiconductor detector, a switch and a control unit. JP-A-2004-125524 also discloses a technique for zeroing the bias voltage applied to the radiation detector after detection of a radiation in order to avoid the polarization.
However, while the bias voltage applied to the radiation detector is zero, the electric charges generated by the radiation detector are not collected. Consequently, gamma rays cannot be measured. Some examinations using a radiological imaging apparatus measure gamma rays emanated from the subject's body to check variations in the gamma rays according to time series. Such examinations are intended for the brain (cerebral infarction, brain tumor, or Alzheimer's disease) and the heart and vessels (myocardial infarction or myocardial ischemia).
Cardiac examinations use, for example, radiopharmaceuticals which are taken and deposited in the normal heart muscle and which are not deposited in a site of the myocardial infarction. Before PET examinations, such radiopharmaceuticals are administered to the subject. A PET apparatus measures gamma rays emanated from the subject's heart muscle to create a tomographic image containing the heart. To create a tomographic image, it is necessary to measure time series data on radiations emanated from the normal parts of the heart and adapt a time-radiation curve for the left chamber lumen and the heart muscle to a compartment model analysis method to determine the blood flow in the local heart muscle. If such time series data is measured, an off time during the gamma ray measurement must be reduced.